High stability clocks and oscillators play an integral role in many modern technologies such as navigation and communications. Laboratory-based primary frequency standards, which utilize microwave transitions between atomic hyperfine levels, provide the highest degree of timing accuracy and are used to form international timescales; in many cases, however, applications beyond timekeeping require clocks that are deployed outside the laboratory setting. One well-known case is that of global navigation satellite systems (GNSS), which employ space-qualified frequency standards aboard satellites in medium earth orbit and/or geosynchronous orbit. While portable clocks are typically outpaced by their laboratory counterparts in terms of stability and accuracy, they nonetheless offer very low levels of frequency instabilities; in the case of rubidium atomic frequency standards, clocks are commercially available with a drift rate below 1×10−13/day and a frequency noise floor less than 1×10−14.
Microwave fountain clocks that incorporate lasers for cooling transitions and utilize a cryogenic sapphire oscillator (CSO) are an ongoing research effort yielding instabilities as low as 1.4×10−14/√{square root over (τ)}. Recently, deployable microwave clocks leveraging a laser cooled Rb have been integrated in satellite systems and others utilizing a pulsed optical pumping routine have shown fractional frequency instability as low as 1.4×10−13/√{square root over (τ)}, with potential to meet constrained size and power requirements for on orbit operation.
With the advent of fully stabilized optical frequency combs, optical frequency standards have rapidly surpassed the capabilities of microwave clocks in both stability and systematic uncertainty. Efforts to reduce the size and increase portability of these systems are an ongoing area of interest. However, these improvements have yet to make an impact on more stringent definitions of portable and deployable clocks, some requirements constraining total clock volume to less than 30 liters. Much of the difficulty in developing compact and environmentally robust optical frequency standards lies with the complicated laser sources and optical systems required for laser cooling and interrogating an atomic sample. Moreover, given the high-quality factor (i.e. narrow spectral linewidth) of typical optical clock transitions, laser pre-stabilization to a high-finesse Fabry-Perot cavity is generally required, which adds significant complexity to the system. Finally, optical frequency combs have historically not been sufficiently compact or robust to warrant an effort toward deployment.
The two-photon transition in rubidium has been described in U.S. Pat. No. 8,780,948 for a precision photonic oscillator, which is a device meant to generate low phase noise microwaves. The oscillator utilizes a “cavity stabilized reference laser” to achieve fractional frequency stability below 5×10−14. However, the cavity stabilization substantially increases the size, weight, complexity, and cost of the system. The nature of an optical cavity is to introduce a length reference to the system that comprises the distance between the mirror or roundtrip distance of the light if there are more than two mirrors. This length scale adds significant sensitivity to mechanical disturbances from acceleration, vibration, and/or thermal expansion.
Space-based atomic frequency standards are critical to the operation of global navigation satellite systems. Conventional space-qualified atomic clocks have several undesirable features including a reliance on specialized parts and manufacturing processes, significant frequency drift, and occasional on-orbit frequency anomalies that lead to increased user range error.